1. Field of the Invention
The present invention also relates to a method of forming a large thread length with thread milling cutter including a shank to be received in a spindle of a machine-tool, and a milling region adjoining the shank at a front end of the shank and including a plurality of rows of teeth extending parallel to each other with the rows of teeth being separated by locking grooves, with the method including a single positioning of the cutter in a bore of a workpiece, adjusting the cutter in an axial direction and feeding it outwardly, and forming the thread by imparting to the cutter a rotational movement, a feeding movement, and a circular movement.
2. Description of the Prior Art
A thread milling cutter of the type described above is well known and is characterized by a uniform distribution of teeth in each row, with all of the teeth. having a complete profile. The length of the milling region corresponds to the large thread length to be formed in a workpiece. The complete profile thread milling teeth are arranged at a distance of pitch P of the to-be-formed thread. The thread milling cutter engages the workpiece along the entire length of its milling region and the thread is formed only by the circular movement. In this case, an increased bending torque acts on the cutter, which can result in the breaking of the cutter.
Also known are thread milling cutters the length of the milling region of which is smaller than the length of a to-be-formed thread, with the teeth still being uniformally distributed in each row and having each a complete profile. This thread milling cutter with a reduced length of the milling region, after forming a portion of a thread and effecting a circular movement, should be repositioned, with an additional circular movement being imparted to the cutter while imparting to it rotational and feeding movements, for forming a further portion of the thread. The repositioning consumes additional or auxiliary time. This is, of course, a drawback, as the additional time increases the manufacturing costs.
Accordingly, an object of the present invention is an improved method of milling large thread length which permits to reduce bending or flexural torque acting on the cutter and to reduce the auxiliary time during forming a thread.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by using a thread milling cutter having a milling section the length of which correspond to the length of the to-be-formed thread and in which at least two adjacent teeth in each row are separated, when viewed in the axial direction of the cutter, by at least one missing tooth space.
With this thread milling cutter, there is no engagement of the missing tooth (teeth) with the workpiece. Therefore, the bending torque, which is caused by the engagement of the milling teeth with the workpiece, is reduced. At that, the additional time necessary for repositioning a cutter, when a cutter with a milling section having a reduced length is used, becomes unnecessary, as an entire length of the thread can be milled. When a first portion of a thread is formed by two teeth, immediately following one another, both are loaded in the same way, while during the formation of the further portion of two immediately following one another teeth, only that tooth, is loaded, which follows in the feeding direction of the circular movement, the missing tooth space.
With the thread milling cutter used according to the present invention, even with several circular movements of the cutter, the only auxiliary time necessary is the time necessary for initial positioning of the cutter which results in with a reduced production time, which is accompanied by reduced manufacturing costs. When conventional thread milling cutters are used, a tangential entry revolution is necessary for dipping of the cutter into the workpiece by an amount of the thread profile depth, and a disengagement revolution is necessary for a clean negotiation of the milled thread grooves. The entry and disengagement revolutions are associated with increased production time. With the thread milling cutter according to the invention, the entry and disengagement revolutions are not necessary.
A thread million cutter, as a rule, is designed only for thread milling However, it is also possible to form a thread milling cutter according to the invention, as a milling and drilling tool having, at its foot end, drilling bits. In this case, the thread milling cutter can be both automatically positioned upon formation of a bore, and positioned in a preformed bore. As a rule, each row of teeth has at least one missing tooth space. Missing tooth space means that a gap is formed between two milling teeth with the gap size corresponding to the dimensions of a milling tooth.
In the thread milling cutter used according to the present invention, the locking grooves can be rectilinear and extend parallel to the cutter axis. As a rule, however, the locking grooves are twisted or have a spiral shape because, together with the formation of gaps between the teeth, the effectiveness of the cutter with twisted locking grooves is improved. The number of the locking grooves or the number of teeth rows is irrelevant for the purposes of the invention and can be arbitrarily selected.
In the used thread milling cutter, in all rows of teeth, the corresponding teeth and the corresponding missing tooth spaces are provided at the same axial positions so that the corresponding teeth lie on a circle the plane of which extend at a right angle to the longitudinal axis of the cutter.
The present invention provides a method of milling large thread lengths which includes providing a thread milling cutter having a milling region the length of which corresponds to the length of the to-be-milled thread and which includes a plurality of rows of teeth which extend parallel to each other, and locking groove means separating the rows of teeth, with the teeth of all of the plurality of rows being arranged in each row one after another, and with at least two adjacent teeth in each row being separated, when viewed in at least one of an axial direction and a circumferential direction, by at least one missing tooth space. The method further includes single positioning of the thread milling cutter in a bore of a workpiece in which the thread is to be milled, with the. cutter being adjusted in an axial direction and fed outwardly, forming the thread by imparting to the cutter rotational feeding and circular movements, and imparting to the cutter, after a circular movement following the single positioning, without repositioning the cutter, an additional circular movement which corresponds to a tooth missing space while the cutter is imparted rotational and feed movements.
However, providing a missing tooth space between two adjacent teeth in each row is not the only important feature of the present invention. It is also important that each tooth, after a positioning, performs its function not only at one complete turn of the circular movement, but also at further complete turns of the circular movement the number of which should correspond to a number of missing teeth in a missing tooth space between two adjacent complete profile teeth. According to the present invention, each tooth is displaced, per each turn of the circular movement, by an axial distance p which corresponds to the distance between a complete profile tooth and the adjacent xe2x80x9cmissingxe2x80x9d tooth. Thus, with the missing tooth space corresponding to one missing tooth, the tooth is displaced over an axial distance 2p.
In the cutter according to one embodiment of the present invention, in each row of teeth, a missing tooth space follows, when viewed in the feeding direction, only one full-profile teeth. However, it is particularly preferable and advantageous when in each row of teeth, a missing tooth space follows, when viewed in the feeding direction, several full-profile teeth. The followed full-profile teeth, if having been worn too much, can be grounded down.
It is further particularly preferable and advantageous when the missing tooth space extends over the entire extension of a missing tooth. In this way, the missing tool space provides for a most possible relief of the thread milling cutter.
It is further preferable and advantageous when the missing tooth space extend over a width of a peak portion of a tooth which amounts at least to 50% of the tooth height. This design is preferred when only a partial relief of the tool is desired. The use of a thread milling cutter with the above-described missing tooth space prevents burr formation in the produced thread.
It is further particularly preferable and advantageous when the missing tooth space has a groove formed in the body of the tool. This groove increases the space between two teeth which limits the missing tooth space. The recessed groove favorable influences formation of sharp free ends of the thread.
It is possible to provide a single missing tooth space in a row of teeth. However, it is particularly preferable and advantageous when each row of teeth has, when viewed in the axial direction, several missing tooth spaces. This contributes to the formation of a tool with a milling or working region having a greater length.